1. Field of the Invention
The present invention relates to an actuator.
2. Description of Related Art
Conventionally, an electromagnetic actuator comprising a coil and a permanent magnet has been generally used as an actuator.
The electromagnetic actuator is an excellent driver which moves largely, but it is difficult to miniaturize such actuator. Therefore, sometimes the actuator cannot be used for a small electric device, such as a watch, where only an actuator having a very limited shape can be mounted because a little space is set aside for the actuator.
In recent years, a piezoelectric actuator which utilizes the piezoelectric effect of a piezoelectric element (hereinafter referred to as “piezoelectric actuator”) has been proposed (for example, JP2008-193893A).
As a piezoelectric actuator, a bimorph type actuator and a unimorph type actuator are commonly used. The bimorph type actuator comprises two plate-shaped piezoelectric elements bonded together. A voltage is applied on one piezoelectric element for extension while a voltage is applied on the other piezoelectric element for contraction so that the actuator bends in a direction perpendicular to the direction in which the piezoelectric elements extends (i.e. direction perpendicular to bonding faces of the piezoelectric elements). The unimorph type actuator comprises a plate-shaped piezoelectric element bonded to one side of a metal plate. A voltage is applied to this piezoelectric element to make it expand and contract so that the actuator bends. The bend (displacement) caused by extension and contraction of the piezoelectric elements is utilized to drive a gear, etc.
However, conventional piezoelectric actuators usually adopted a plate-shaped piezoelectric element which comprises electrode layers bonded on both sides of a piezoelectric ceramic material such as lead zirconate titanate (PZT). Therefore the bend (displacement) according to extension and contraction of the piezoelectric elements is small. Also, these actuators have further problems—for example, a high voltage is needed.
In recent years, a plate-shaped thin electroconductive polymer actuator has been developed. This actuator comprises an electrolyte layer between electrode layers. When a voltage is applied to the electroconductive polymer actuator, ions move in the electrolyte layer and molecules around the electrodes expand so that the actuator physically changes its shape causing bend (displacement) at its plate-shaped portion.
In such configuration of the actuator, the plate-shaped portion is displaced by application of a relatively low voltage. In addition, the configuration is simple, and bend (displacement) of the plate-shaped portion is relatively large. Such actuator is expected to be an efficient driver.